Compare Model Drawings, CAD & Specs Type Blaze Wavelength Primary Wavelength Region Peak Efficiency Grooves per mm Availability Price
77740
$1,025

Features

High Quality Richardson Diffraction Gratings

Plane ruled and holographic gratings listed here are fabricated from float glass substrates with an aluminum coating. The Oriel monochromators and spectrographs feature diffraction gratings produced by Richardson Gratings. Both Oriel Instruments and Richardson Gratings are part of the Newport family of brands, and have a long history of working together to design instruments that are appropriate for a wide variety of applications.

Selecting a Diffraction Grating

Diffraction gratings are primarily selected based on the spectral resolution requirements of the application and the spectral region of interest.

Spectral Resolution

Diffraction gratings are available in various groove densities (i.e. lines/mm). Higher groove densities give higher reciprocal dispersion and therefore higher resolution. The grating dispersion is similar for gratings with the same groove density. The exact dispersion is dependent upon other physical characteristics of the grating in addition to the groove density.

The resolution is the ability to separate wavelengths. It is usually expressed as the Full Width Half Maximum (FWHM). The resolution can be theoretically determined by multiplying the reciprocal dispersion of the grating by the slit width. The monochromator bandpass with a 1200 lines/mm grating is half that of the same arrangement with a 600 lines/mm grating. Note that this simple relationship is not accurate for slit widths below 50 µm, as the optical aberrations begin to play a role in the resolution.

Using a grating with a high groove density may increase resolution, but the spectral range narrows. The dispersion of a grating changes inversely with the groove density. If the groove density is halved, the dispersion is doubled. When performing a scan, to save time it is important to consider the resolution when determining the interval wavelength (i.e. the step size) of the scan. For example, if the resolution with a particular grating and slit is 5 nm, it is not necessary or practical to perform a scan every 1 nm.

Typical output power and resolution of various Oriel Tunable Light Sources, which utilize a Cornerstone 130 monochromator with extended range gratings. The slit width is set to 120 µm in the above illustration.

Spectral Region of Interest

The Blaze Wavelength is the wavelength for which a blazed diffraction grating is most efficient a diffracting monochromatic light into the first order.  Choosing a blaze wavelength that is close to the spectral region of interest will allow for the highest possible efficiency.

High-efficiency gratings are desirable for several reasons. A grating with high efficiency is more useful than one with lower efficiency in measuring weak transition lines in optical spectra. A grating with high efficiency may allow the reflectivity and transmissivity specifications for the other components in the spectrometer to be relaxed. Moreover, higher diffracted energy may imply lower instrumental stray light due to other diffracted orders, as the total energy flow for a given wavelength leaving the grating is conserved (being equal to the energy flow incident on it minus any scattering and absorption).

Plane Ruled Diffraction Gratings

For a plane blazed grating, the groove spacing and blaze angle determine the distribution of energy. The blaze direction for most gratings is specified for first order Littrow use. In Littrow use, light is diffracted from the grating back toward the source. Gratings used in the Littrow configuration have the advantage of maximum efficiency, or blaze, at specific wavelengths.

Holographic Gratings

Holographic gratings normally have a sinusoidal groove shape, which is the result of recording interference fringe fields in photoresist material. Since the grooves are symmetrical, they do not have a preferred blaze direction and hence the gratings carry no blaze arrows. The range of useful diffraction efficiency is controlled by varying the modulation (the ratio of groove depth to groove spacing). The lower the modulation, the shorter the wavelength limit to which the grating can be used, but the peak efficiency may be lowered as well. We have found that three modulation levels are adequate for nearly all purposes.

Special Orders Welcome

In addition to the gratings listed here, special order gratings may also be available for use in a wide variety of applications. Custom grating requests include gold coating for increased IR efficiency, AlMgF2 coating for increased UV efficiency and many other requests. Gratings may be installed into a variety of mounts for use in specific monochromators or spectrographs. Contact Newport Sales for more information.

Resources

Technical Notes